part of it was that Floyd played the same show day in and day out so it was easier to know exactly what to expect musically each day. Easier to have effects in place when you know exactly what is coming next, and you rehearsed it that way for months before taking it on the road.
This is the closest answer that I can live with. To the point I have to agree all the way about doing the same show / the same effects.That aside The Dead still could have utilised the surround effect for their benifit and to even go further they did eventually have delay towers for the bigger venues post WOS P.A. set-ups.